Interfacial microstructure and mechanical properties of

Interfacial microstructure and mechanical properties of aluminium –zinc-coated steel joints made by a modified

metal inert gas welding –brazing process

H.T.Zhang a,?,J.C.Feng a ,P.He a ,H.Hackl b

a

State Key Laboratory of Advanced Welding Production Technology,Harbin Institute of Technology,Harbin 150001,

Heilongjiang Province,PR China

b

Fronius.Internation GMBH,A4600Wels-Thalheim,Austria

Received 10May 2006;accepted 4July 2006

Abstract

The microstructure and properties of aluminium –zinc coated steel lap joints made by a modified metal inert gas CMT welding –brazing process was investigated.It was found that the nature and the thickness of the high-hardness intermetallic compound layer which formed at the interface between the steel and the weld metal during the welding process varied with the heat inputs.From the results of tensile tests,the welding process is shown to be capable of providing sound aluminium –zinc coated steel joints.?2006Elsevier Inc.All rights reserved.

Keywords:Welding –brazing;Heat input;Intermetallic compound

1.Introduction

In order to reduce pollution and save energy,it is attractive to make car bodies lighter by introducing some aluminium parts as substitutes for the previous steel structures [1,2].Therefore,joining aluminium to steel has become a major problem,requiring resolution.Direct solid-state joining can be used to make these dissimilar metal joints by controlling the thickness of the interme-tallic compound layer that develops within a few micrometers of the joint interface [3–9].However,the shape and size of such solid-state joints are extremely restricted.Thus,the joining of aluminium to steel by

fusion welding methods has been widely studied.As is well known,the joining of aluminium to steel by fusion welding is difficult because of the formation of brittle interface phases which can deteriorate the mechanical properties of the joints.However,Kreimeyer and Sepold [10]have shown that if the layer is less than 10μm thick,the joint will be mechanically sound.In addition,the authors also deem that the existence of a zinc coating increases the wettability of the Al to the steel substrate.As another approach,Achar et al.[11]reported that the thickness of the intermetallic compound layer formed during TIG arc welding of Al to steel is decreased by the use of an Al alloy filler metal containing Si.Murakami et al.[12]and Mathieu et al.[13]both point out that the temperature probably determines the thickness of the intermetallic compound layer of the joint and recom-mended the use of lower heat input to obtain a sound joint.

Materials Characterization 58(2007)588–

592

?Corresponding author.Tel.:+8645186412974;fax:+8645186418146.

E-mail address:hitzht@https://www.360docs.net/doc/4d898264.html, (H.T.Zhang).1044-5803/$-see front matter ?2006Elsevier Inc.All rights reserved.doi:10.1016/j.matchar.2006.07.008

The cold metal transfer process,identified here as CMT,is a modified metal inert gas welding process which invented by the Fronius Company.The principal innovation of this method is that the motions of the welding wire have been integrated into the welding process and into the overall control of the process.Every time the short circuit occurs,the digital process-control both interrupts the power supply and controls the re-traction of the wire.The wire retraction motion assists droplet detachment during the short circuit,thus greatly

decreasing the heat input during welding.

In this study,we selected the CMT process to join aluminium to zinc-coated steel using a lap geometry. The main purpose of this effort was to reveal the rela-tionship between heat input and the microstructure of the joint.Hardness testing was also used to characterize the phases formed during the welding process.In ad-dition,the quality of the joints was assessed by tensile testing.

2.Experimental

Deep drawn sheets of hot-dip galvanized steel and sheets of pure Al1060with thickness of1mm were used in the welding experiments.An Al sheet was lapped over a Zn-coated steel sheet on the special clamping fixture, and the ending of the weld wire was aimed at the edge of the aluminium sheet,as shown in Fig.1.The MIG welding–brazing was carried out using the CMTwelding source with an expert system and1.2-mm-diameter Al–Si filler metal wire.Argon was used as the shielding gas at a flow rate of15L/min.The surface of the samples was cleaned by acetone before welding.Two sets of welding parameters of different heat inputs were selected,as shown in Table1.The heat input,J,is calculated using the equation:J=(60×UI)/v,where U is the mean welding voltage,I is the mean welding current and v is the welding speed.

Typical transverse sections of the samples were observed using optical microscopy(OM)and scanning electron microscopy(SEM).The composition of the intermetallic compound layer at the interface between the steel and the weld metal was determined by energy dispersive X-ray spectroscopy(EDX).Hardness values were obtained using a microindentation hardness tester with a load of10g,and a load time of10s.In addition, the samples were cut in10mm widths,and transverse tensile tests(perpendicular to the welding direction) were used to measure the joint tensile

strength.

Fig.1.Schematic plan of the welding process.

Table1

The welding parameters

Sample

number

Mean

welding

current

(A)

Mean

welding

voltage

(V)

Wire feed

rate

(m/min)

Welding

speed

(mm/min)

Weld

heat

input

(J/cm)

Sample A6611.8 3.9762613.2

Sample B11013.3 5.4913

961.5

Fig.2.Front(upper)and back(lower)appearances of typical joints

with different heat inputs:(a)Sample A;(b)Sample B.

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H.T.Zhang et al./Materials Characterization58(2007)588–592

3.Results and discussion 3.1.Macro-and microstructures

The appearance of the weld seams for different heat inputs are shown in Fig.2.For all welding cases,a smooth weld seam was made.The molten metal wetted the steel better when using lower heat input,i.e.,compare Sample A at lower heat input to Sample B.This may be related to the different degree of evapo-ration of the zinc coating at different heat inputs.While improving the heat input,the greater evaporation of zinc reduces the wettability of the molten metal on the steel.

Fig.3shows a typical cross-section of the joints.Higher heat input (Sample B)resulted in a decrease in the contact angle between the steel and the weld metal.Meanwhile,a special zone with lighter colour at the toe of the weldments can be found (designated by white arrows in Fig.3).Optical micrographs shows that a visible intermetallic compound layer has formed be-tween the steel and weld metal during the welding process,Fig.4.The thickness of the intermetallic com-pound layer changes not only with the location within a given joint but also with the varying heat input between different joints.The thickness of the intermetallic compound layer in the center is greater than at the edge of the seam within one joint.For Sample A,the maximum thickness of the compound layer is about 10μm but is 40–50μm for Sample B.

The microstructure of the intermetallic compound is shown in greater detail in the SEM micrographs in

Fig.5.At lower heat input (Sample A),the inter-metallic compound presents a serrated shape oriented toward the weld metal.When the heat input was increased (Sample B),the compound layer became much thicker and grew into the weld metal with tongue-like penetrations.Anisotropic diffusion is a possible explanation for this irregularity.The intermetallic compounds that form under these conditions generally have an orthorhombic structure (see below).Because of the high vacancy concentration along the c -axis of the orthorhombic structure,Al atoms can diffuse rapidly in this direction and cause rapid growth of the inter-metallic compound.

EDX analysis was used to determine the phases of the intermetallic compound layer.The results show that the intermetallic compound layer of the joint made by lower heat input consists entirely of Fe 2Al 5.But when the heat input is increased,the intermetallic compound layer consists of two different phases,the FeAl 2phase near the steel surface and a FeAl 3phase which penetrates toward the weld metal.Thus it is clear

that

Fig.4.Optical microstructures of interface between steel and weld metal:(a)Sample A;(b)Sample

B.

Fig.3.Cross-section image at limit of penetration in the joint,showing change in contact angle with increased heat input.Arrows point to an intermetallic compound at the tip of the weld metal:(a)Sample A;(b)Sample B.

590H.T.Zhang et al./Materials Characterization 58(2007)588–592

the intermetallic compound layer that forms is closely related to the heat input during the welding process.

With regard to the special zone designated by white arrows in Fig.3,dendritic-appearing structures can be distinguished on a high-magnification SEM micrograph (Fig.6).EDX analysis results show that such dendrite-shaped crystals of an Al-richα-solid solution containing residual zinc routinely formed at this location.

3.2.Hardness measurements

Hardness testing results also confirm the presence of a hard intermetallic compound layer.The hardness of the interface layer is much higher than that of the base metal and the weld metal and is found to vary for the corresponding intermetallic compound phases.For the high heat input weld(Sample B)the hardness is much higher,Fig.7

.

Fig.8.The location where the fracture occurred during tensile testing (designated by white arrows):(a)Sample A;(b)Sample

B.

Fig.7.Microindentation hardness test results of the joints made using different heat

inputs.

Fig.6.Dendrite crystal structure at the toe of the weldment(Sample

B).

Fig.5.SEM micrograph of interface between steel and weld metal:(a)

Sample A;(b)Sample B.

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3.3.Tensile test results

The tensile tests were performed to provide a qualitative measure of the joint strength and behavior. These results show that the bond strength is excellent, with the fractures occurring in the HAZ of the Al even when the thickness of the intermetallic compound layer was greater than40μm,Fig.8.From a general view-point,the thickness of the intermetallic compound layer should be controlled to less than10μm in order to obtain a sound joint.This implies that the joint made with higher heat input should have a lower intrinsic strength than the other because of the thicker brittle intermetallic compound layer.However,the intrinsic strength of the joints cannot be determined when the fracture occurs in the HAZ of the pure Al.Nevertheless, according to the thickness of the compound layer,we can presume that the intrinsic strength of the joints should be decreased when increasing the welding heat input.

4.Conclusions

Based on the experimental results and discussions, conclusions are drawn as follows

1)Dissimilar metal joining of Al to zinc-coated steel

sheet without cracking is possible by means of a modified metal inert gas(CMT)welding–brazing process in a lap joint.

2)Fe–Al intermetallic compound phases were formed

at the interface between the steel and the weld metal.

The thickness and the composition of the interme-tallic compound layer varied with weld heat input.

3)Despite the formation of the intermetallic compound

phases,the interface between steel and weld metal is not the weakest location of the joints.Tensile tests of the joints caused fractured in the Al HAZ,even when the intermetallic compound layer thickness exceeded 40μm.Acknowledgements

The authors wish to acknowledge the financial support provided by the National Natural Science Foundation under Grant No.50325517for this work. References

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